16.39 ). In addition, the ejection fraction, which is the ratio of the volume of blood ejected from the left ventricle per beat (stroke volume) to the volume of blood in the left ventricle at the end of diastole (end-diastolic volume), is widely used clinically as an index of contractility.
•Fig. 16.38 Leftventricularpressurecurveswithtangentsdrawntothesteepestportionsoftheascendinglimbstoindicatemaximalratesofchangeinpressurewithtime(dP/dtvalues).A(blue curve), control;B(dashed red curve), hyperdynamicheart,aswithadministrationofnorepinephrine;C(green dashed curve), hypodynamicheart,asincardiacfailure.160 120 80 40 0 0 0.2 0.4 0.6 Time (sec) Left ventricular pressure (mm Hg) Max dP/dt B A C
The atria are thin-walled, low-pressure chambers that function more as large-reservoir conduits of blood for their respective ventricles than as important pumps for the forward propulsion of blood. The ventricles comprise a continuum of muscle fibers originating from the fibrous skeleton at the base of the heart (chiefly around the aortic orifice). These fibers sweep toward the cardiac apex at the epicardial surface. They pass toward the endocardium and gradually undergo a 180-degree change in direction to lie parallel to the epicardial fibers and to form the endocardium and papillary muscles.
At the apex of the heart, the fibers twist and turn inward to form papillary muscles. At the base of the heart and around the valve orifices, these myocardial fibers form a thick, powerful muscle mass that not only decreases the ventricular circumference to implement the ejection of blood but also narrows the AV valve orifices, which aids in closure of the valve. Ventricular ejection is also accomplished by a decrease in the longitudinal axis as the heart begins to narrow toward the base. The early contraction of the apical part of the ventricles, coupled with the approximation of the ventricular walls, propels the blood toward the ventricular outflow tracts. The right ventricle, which develops a mean pressure that is approximately one seventh that developed by the left ventricle, is considerably thinner than the left ventricle.
The cardiac valve leaflets consist of thin flaps of flexible, tough, endothelium-covered fibrous tissue that are firmly attached at the base to the fibrous valve rings. Movement of the valve leaflets is essentially passive, and the orientation of the cardiac valves is responsible for the unidirectional flow of blood through the heart. There are two types of valves in the heart: atrioventricular and semilunar (
Figs. 16.40 16.41
The tricuspid valve, located between the right atrium and the right ventricle, is made up of three cusps, whereas the mitral valve, which lies between the left atrium and the left ventricle, has two cusps. The total area of the cusps of each AV valve is approximately twice that of the respective AV orifice, and so considerable overlap of the leaflets occurs when the valves are in the closed position. Attached to the free edges of these valves are fine, strong ligaments (chordae tendineae cordis) that arise from the powerful papillary muscles of the respective ventricles. These ligaments prevent the valves from becoming everted during ventricular